I know this is an old thread, but it's still relevant for parents of infants driving an i3.
We have successfully mounted a Graco SlimFit3 LX 3 in 1 Car Seat in rear facing mode. Sure it requires the front seat to be moved forward, but not excessively so, especially on the passenger side.
It's hard to take a child in and out due to the setup of the i3 rear doors, but it's possible and we've been making it work for almost a year.
I am on the lookout for a second car seat now and if I cannot find anything better fitting in rear facing mode, we'll end up buying another Graco SlimFit3 LX. It's a very usable seat.
Current daily driver: 2017 BMW i3 Rex
Past car history: Nearly a dozen 85-89 Toyota MR2s including 88 Supercharged, 96 Lexus SC300, 05 Tacoma 4x4 manual, 11 Golf TDI, 01 Honda Insight manual